A storage system is provided with multiple storages, wherein a backup is made by copying one storage (working storage) to another storage (backup storage).
Additionally, a remote copy function has been known wherein data in one storage system of multiple storage systems remote from each other is copied to another storage system.
For example, in a synchronous copy mode, write of data to a source storage system is executed synchronously with write of data to a destination storage system, for achieving the mirroring between remote sites.
Patent Reference 1: Japanese Laid-open Patent Publication No. 2006-260292
In such conventional remote copy techniques for a storage system, however, information about the difference status of a copy from a working storage to a backup storage in a source storage system may not be migrated to a destination storage system, which is inconvenient.